The invention will be described in connection with the cleaning of tin-plated surfaces, such as tin cans, which are soiled with organic lubricants that are applied to the tin surface as drawing aids during cold forming operations. The invention has, nevertheless, broader applicability.
Cleaning is essential as a preliminary to many surface finishing operations. It is normally required, for example, prior to corrosion preventive treatments and prior to the application of organic finishes to the surface, and is especially important in the case of metal surfaces to which organic materials have been recently applied as an aid to cold forming. These materials must be removed in order to obtain a surface suitably receptive to an organic or inorganic finish.
One example of the need for such cleaning is in the manufacture of two-piece, tin-plated, drawn and ironed cans. Circular blanks of tin-plated steel, which, due to the high cost of tin, generally have only a thin layer of tin plating, are first cupped and then passed through several drawing dies to iron the cup in order to form a unitary side wall and can bottom structure. A can produced by this process may be characterized as a thin-walled, thick-bottomed container having a generally uniform wall thickness. Such cans will be referred to herein as "DI" (drawn and ironed) cans. These forming operations are assisted and the dies and metallic surface protected by the application of lubricants to the tin-plate surface prior to or during the forming operation. The lubricants that are deposited on the tin surface usually consist of various types of mineral and vegetable oils and heavy metal soaps. Since it is desired to have a clean surface in order to assure adhesion of a subsequently applied sanitary lacquer and/or decorative varnish, the cleaning step after forming is critical to a successful manufacturing process.
A serious problem with DI tin-plated containers is that the drawing operation stretches the tin-plate surface, thereby exposing some of the underlying metal, which exposure may lead to corrosion. The underlying metal may be a ferrous metal, such as iron, iron alloys, and a wide variety of steels.
A problem with cleaners for tin-plate has been to obtain a cleaner that will provide a water-break-free surface without unduly etching the tin and without promoting corrosion of the underlying metal. A water-break-free surface is a surface that is sufficiently freed of lubricants, soil, and other contaminants so that it will maintain a continuous film of water.
Etching results from chemical attack of the cleaner on the tin surface and results in a roughened and dull surface. Furthermore, etching removes a portion of the corrosion protective tin from the surface, thereby degrading the anti-corrosion qualities of the surface and aggravating the problem of corrosion due to exposure of the underlying metal. Where a smooth, shiny surface is desired, such as in the case of a beverage can, etching and/or corrosion is clearly undesirable.
Another problem with cleaners for tin-plate has been that they do not protect against conditions on the processing line where, after drawing and ironing and cleaning, line stoppage can expose the cleaned cans to the cleaner for prolonged periods of time. This prolonged exposure can lead to corrosion of the underlying surface exposed due to stretching or due to etching, thereby rendering the containers unacceptable for use. Additionally, any corrosion and blemishes on the surface will adversely affect the adhesion of any conversion coating or sanitary lacquer coating that is applied thereafter.
Alkaline cleaners, such as aqueous solutions of alkali metal salts of silicates, phosphates, carbonates and borates, which have been found to be most suitable for producing the desired water-break-free surface, possess the undesirable tendency to etch the surface. It has previously been discovered that the addition of certian compounds to the aqueous alkaline cleaning solutions will help to inhibit etching of the tin surface under the alkaline conditions employed.